1. Field of the Invention
This invention relates to a method of upgrading the quality of a compound semiconductor crystal exemplified typically by a semi-insulating gallium arsenide substrate crystal used for a gallium arsenide integrated circuit.
2. Description the Prior Art
In semi-insulating substrates crystals used for gallium arsenide integrated circuits, a crystal grown by the melt pulling method (LEC method) has crystal microdefects, i.e. dislocation density of generally ranging from 2-3.times.10.sup.4 cm.sup.-2 to 5.times.10.sup.5 cm.sup.-2 distributed unevenly in crystallographical (100) plane. Lately, it has been reported that the distribution of dislocations is related with the characters of a field effect transistor of gallium arsenide, in particular, the ununiformity of threshold voltage Vth to determine "ON-OFF" of swithcing (e.g. Miyazawa: "Oyo Butsuri (Applied Physics)" Vol. 52, N0. 3 (1983)). Thus, various efforts have been made to obtain crystals with a lowered dislocation density or uniform distribution of dislocation density by improving the temperature condition during growth thereof, but it has not been realized sufficiently to realize a uniform dislocation distribution.
Moreover, in LEC crystals, there are cellular dislocation networks of several hundred microns in diameter and it is found by cathodoluminescence that the crystalline quality near the cellular wall of the dislocation network is different from other regions (e.g. Chin et al: "J. Electrochem. Soc." Vol. 129, No. 10 (1982), page 2386-2388). The presence of this dislocation network leads also to occurrence of ununiform characters of a transistor. However, the dislocation network is incorporated or formed during the growth of a crystal and accordingly, it is difficult to eliminate the dislocation network.
As described above, the form of an arrangement of dislocation in an LEC crystal has influences upon the dynamic characteristic of a transistor and constitutes a bar to realization of an integrated circuit on a wafer. Of course, it has eagerly been desired to overcome this drawback, but the inventors do not know an effective prior method of making a uniform compound semiconductor crystal.